Recent industrial advancement has required high purity and high quality of products, and thus a separator (or a membrane) technology has been recognized as a very important field. In the environmental sector, especially as the need for clean water and the awareness of a lack of water increases, a technology of using a membrane has largely attracted much attention as one of ways to solve these problems. Processes such as water purification, sewage, waste water, and desalination using a membrane, are already rapidly spreading. In addition, the membrane technology has been developed for applications away from the membrane itself, and has expanded into surrounding technology development and has also enhanced membrane performance improvement according to applications as well.
The membrane is a substance having a selection capability that is present between two different materials. In other words, the membrane means a substance which serves to selectively pass or to exclude a material. Structures and substances of the membrane, and conditions and principles of the movement of the materials passing through the membrane, have no limitations. When a substance is located between only two materials to isolate the two materials each other and the selective movement of the materials through the substance between the two materials occurs, the substance may be called a membrane.
The membranes are of a very variety of types and are classified into several criteria.
First, a classification by a separation operation is a classification method depending upon the state of a target material to be separated, and is classified into a liquid separation method, a gas-liquid separation method, a gas separation method, and so on. The liquid separation method is classified into micro filtration, ultra filtration, nano filtration, reverse osmosis filtration, etc., in accordance with the size of an object for filtration. The gas separation method is classified in detail in accordance with the type of gas to be separated. The gas separation membrane is classified into an oxygen-enriched membrane for separating the oxygen gas, a nitrogen-enriched membrane for separating the nitrogen gas, a hydrogen-enriched membrane for separating the hydrogen gas, a dehumidifying film for removing humidity, etc.
The membrane is classified according to a film-like shape, and is classified into a flat membrane, a hollow fiber membrane, a tubular membrane, etc. In addition, the membrane is also classified into a plate-shaped type, a spiral wound type, a cartridge type, a flat membrane cell type, an immersion type, a tubular type, and so on, depending on the form of a filter module.
The membrane is classified according to a material and is classified into an inorganic film and an organic film using a polymer film. In recent years, however the inorganic films expand their use based on the advantages of heat resistance, durability, etc., most currently commercialized products are occupied by the polymer membranes.
In general, filtration means to remove two or more components from a fluid, that is, it means to separate non-dissolved particles (that is, solid) from the fluid. Filtering mechanisms in the separation of the solid materials may be described as sieving, adsorption, dissolution, diffusion mechanisms. Except for some membranes such as gas separation membranes, reverse osmosis membranes, etc., it can be said that most of the filtering mechanisms depend entirely on the sieving mechanism.
Therefore, it is possible to use any materials with pores as filter media. Nonwoven fabrics (nonwovens), woven fabrics (wovens), meshes, porous membranes and the like are typical filter media.
It is difficult to make pores not more than 1 μm in the case of nonwovens, wovens, meshes, etc. Thus, the nonwovens, wovens, meshes, etc., are used as a pretreatment filter concept with a limitation to a particle filtration area. Meanwhile, porous membranes can make precise and small pores and have been used for a process requiring a wide range of filtration areas and the highest precision such as micro filtration, ultra filtration, nano filtration, reverse osmosis filtration, etc.
Since the nonwovens, wovens, or meshes are made of fibers having a thickness from several micrometers to several hundreds of micrometers, it is difficult to make fine pores not more than 1 μm. In particular, it is not possible actually to create uniform pores since webs are formed by random arrangement of fibers in the case of the nonwoven fabrics. The melt-blown nonwoven fabric may be called a nonwoven fabric made of a very fine fiber having a diameter of 1˜5 μm. The pore size before heat calendering is not less than six micrometers and the pore size after heat calendering is only three micrometers approximately. The deviation in the average pore size occurs by ±15% or more around a reference point, and the melt-blown nonwoven fabric has a structure in which very large pores coexist. Accordingly, the nonwovens, wovens, or meshes have the difficulty in preventing the leakage of contaminated materials through relatively large pores and thus have low filter efficiency. Therefore, the filter media are used in an inaccurate filtration process or used as a pre-treatment concept of an accurate filtration process.
Meanwhile, the porous membrane is prepared by a method such as a non-solvent induced phase separation (NIPS) process, a thermally induced phase separation (TIPS) process, a stretching process, a track etching process, a sol-gel process, etc. The materials of most of the porous membranes are made of representative organic polymers, such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), nylon (Nylon6 or Nylon66), polysulfone (PS), polyethersulfone (PES), polypropylene (PP), polyethylene (PE), nitrocellulose (NC) or the like. While the conventional porous membranes may create pores of precise and small size, closed pores or blinded pores may be created inevitably in the manufacturing process. As a result, the conventional porous membranes have problems such as a small flow amount of filtration, a high driving pressure, and a short filtration lift cycle, to thus cause high operating costs and frequent filter replacement.
Therefore, it is in an urgent situation to develop a membrane of a long lifetime and a high efficiency having constant filtering performance and stability according to the pore size as a thin film of a micro-porous structure so as to be used for the liquid treatment.
Korean Patent Application Publication No. 2008-60263 (Patent Document 1) discloses a filtering medium including at least one nanofiber layer of polymeric nanofibers having an average fiber diameter of less than about 1 μm, in which a mean flow pore size is about 0.5 μm to about 5.0 μm, and also that a solidity is about 15% by volume to about 90% by volume, and a water flow rate through the medium at a differential pressure of 10 psi (69 kPa) exceeds about 0.055 L/min/cm2.
The method of manufacturing a filtering medium proposed in Patent Document 1 discloses spinning nanofibers by an electro-blown spinning method or an electric blowing method by using a solution containing nylon of 24 wt % in a formic acid as a polymer solution, including at least one spinning beam comprising spinnerets, blowing gas injection nozzles and collectors, to thereby form webs.
However, the method of forming a fibrous web of nanofibers in the patent document 1 cannot be referred to a manufacturing technique that uses a multi-hole spinning pack. In addition, when manufacturing a nanofiber web by an electrospinning method in an electrobrown spinning apparatus using a multi-hole spinning the pack in order to increase productivity, a spinning solution containing a polymer of 24 wt % increases the viscosity and is solidified at the surface of the spinning solution, to thus make it difficult to spin for a long time, and increases the fiber diameter, to thus cause a problem that it is not possible to make the fibrous form of not more than a micrometer in size.
Furthermore, in the case that the ultrafine fiber web obtained by electrospinning does not go through a pretreatment process of appropriately adjusting the amount of the solvent and moisture remaining on the surface of the web before performing calendering, pores are increased but the strength of the web is weakened. Otherwise, in the case that evaporation of the solvent is performed too slowly, a phenomenon of melting the web may occur.
Meanwhile, Korean Patent Application Publication No. 2012-2491 (Patent Document 2) discloses a filter medium for a liquid filter, a method of manufacturing the same, and a liquid filter using the same, using an electrospun nanofiber web to have a multi-layer structured three-dimensional micro-porous structure to thereby have high efficiency and long lifetime and maximize filter efficiency.
The filter media made of a multi-layer nanofiber web for liquid filters are prepared by air-electrospinning a spinning solution on top of a support to thus form a nanofiber web, in which the spinning solution is obtained by dissolving a fiber forming polymer material in a solvent, thermo-compression bonding the support in which the nanofiber web has been formed, or air-electrospinning a spinning solution to thus form and thermo-compression bond a nanofiber web, to then laminate the support on one side of the thermo-compression bonded nanofiber web.
However, the process of thermo-compression bonding the support in which the nanofiber web has been formed after air-electrospinning the spinning solution on top of the support to thus form a nanofiber web, in the method of preparing the filter media for liquid filters, may use a porous nonwoven fabric having a high tensile strength as the support, to thereby increase the tensile strength and have the benefit of increasing the ease of handling during the production but may cause a problem that uniformity of the nanofiber web is not high.
Generally, the electrospun nanofibers are integrated in a collector, that is, bring about an integration phenomenon, and laminated along a pattern of an integrated portion, that is, bring about a lamination phenomenon. For example, when electrospinning is executed on a diamond pattern, nanofibers starts to be integrated along a first diamond pattern.
Therefore, in the case of forming a nanofiber web by spinning nanofibers directly on a nonwoven fabric, as disclosed in Patent Document 2, there exist problems that a nanofiber web having excellent uniformity cannot be obtained in view of a nanofiber web pore size, permeability, thickness, weight, and so on.
In Patent Document 2, was proposed filter media that are laminated with a nanofiber web by using a nonwoven fabric made of a fiber with a single core structure such as a melt-blown nonwoven fabric, a spun bond nonwoven fabric, and a thermal bond nonwoven fabric, but it is difficult to maintain a pore structure in any one of the nonwoven fabric and the nanofiber web since calendering is performed at a relatively lower melting point of melting points of the nonwoven fabric and the nanofiber web when the nonwoven fabric and the nanofiber web are laminated on each other.
Further, as disclosed in Patent Document 2, in the case of making filter media by air-electrospinning a spinning solution to thus form nanofiber web, thermo-compression bonding the nanofiber web, and then laminating a support on one surface of the thermo-compression bonded nanofiber web or in the case of making filter media with only nanofibers by themselves, the filter media of a heavy weight of about 10 g/m2 or more are required in order to handle the filter media. However, the heavy-weight of the filter media is a factor directly connected with a production rate, to thus cause high costs.
In addition, nanofibers have a large amount of static electricity in a manufacturing process, and thus when the filter media include only nanofibers themselves, it is quite difficult to handle the filter media. It is not possible to remove the static electricity through a composition such as lamination, but it is possible to improve handling properties. Furthermore, although nanofibers may have good relative intensities as compared with the other fibers, the absolute strengths of nanofibers are prone to be weak.
In addition, a porous nanofiber web made of nanofibers may create a rigid coupling between the fibers through a calendering process, to thus create a highly matured porous nanofiber web. However, when performing a calendering process by spinning a spinning solution directly onto a nonwoven fabric as disclosed in Patent Document 2, the melting point of the nonwoven fabric is lower than an inter-fiber bonding temperature of the nanofibers made of a polymer, to accordingly limit a calendering temperature control. As a result, a rigid coupling between the nanofibers to form the nanofiber web cannot be made.
The property of hydrophilicity is required in the liquid filtration. However, when preparing the filter media by using the hydrophilic polymer, there is a problem that the hydrophilic polymer is weak in the mechanical strength and the chemical resistance as compared to the hydrophobic polymer and thus should be used limitedly.
As a result, the filter media made of a PVdF polymer have a very suitable strength and chemical resistance to liquid filtration, but there is a problem that the filter media are limitedly used in aqueous filtration because of the hydrophobic property. In addition, in the case that the filter media made of a hydrophobic polymer are made of a heavy weight of about 10 g/m2 or more so as to be handled in the manufacture process, a relatively high differential pressure is applied across the filter media when an aqueous fluid passes through the filter media. Accordingly, in the case that the hydrophilic treatment is performed or not, there is a problem that water does not pass through the filter media well if an appropriate force is not applied across the filter media.
In addition, in the case of making the filter media made of only nanofibers themselves of a heavy weight of about 10 g/m2 or more, a thin filter layer cannot be formed. Thus, since a high differential pressure is applied across the filter media, there is a problem that a pass flow rate becomes little.
In general, methods of treating water pollutants may include a co-precipitation method of using a waste water treatment coagulant, a flotation method according to a specific gravity difference, a bioaccumulation method, an ion exchange adsorption method, etc. However, the ion exchange adsorption method has been known as the most effective method from among them.